I have an Long Tent. Anemone that has been doing fine for months.
Growing, nice color, eats well, seems very happy.
Went away for a few days and when I came back it was almost completely
white - about 1/4 of it's oral disk is still brown. No damage
noticable. Tank is established for about 4 years - no other issues -
no changes - good lighting - excellent water conditions.

Possibility: it is close to an octopus frogspawn that is growing
pretty quickly - would a sting do this?